The demand for telephone communication systems has substantially increased in recent years. In order to meet this demand, it is necessary that the capacity of telephone transmission links also increase. There are two well known techniques by which the system capacity can be increased, one is to physically increase the number of available channels on a transmission route and the second is to increase the bandwidth of the existing channels. In the case of microwave radio links the former technique requires building new antenna towers and/or adding more antennas to existing towers while the latter technique can be implemented by switching to a higher frequency with a wider bandwidth. The latter technique is clearly the most economical choice, however, transmission at the higher frequencies require that the movement of an antenna be maintained within predetermined limits at the microwave frequency of interest.
Such movement has been measured using conventional ground surveying equipment such as a transit or theodolite. Such techniques, while they can give a rough estimate of motion, are not suitable for dynamic measurements especially in high winds where motion of the surveying equipment may be as much of a problem as the movement of the tower.
An additional approach to the problem is disclosed in an article titled "Deflection of Tall Guyed Towers" by J. K. Groseclose in Civil Engineering, November 1964, pages 68 to 70. The article describes an instrument which monitors the translational motion of a tower using a ground mounted laser which directs a light beam towards the upper portion of the tower. This fixed beam is observed by a servo-driven photocell system movably mounted on the tower. Such a system requires complicated servo drive apparatus to detect the fixed beam as the tower moves.
Heretofore, estimates of tower movement under various loadings (antennas, wind, etc.) were accomplished using theoretical models since there is no empirical data available on full size towers due to the lack of equipment capable of dynamically monitoring such movement.
Accordingly, there is a need for a technique for accurately measuring the motion (e.g., horizontal motion, twist) of a microwave tower or the like.